The use of polymeric alkyl cyanoacrylates as resist materials for microlithography is known in the art. For example: IBM Technical Disclosure Bulletin, Vol. 16, No. 11, April 1974, p. 3745, discloses the use of poly methyl alpha cyanoacrylate homopolymers and copolymers as high-speed positive electron-beam resists; Japan Pat. No. 80/21,332 discloses the use of poly butyl cyanoacrylate as a positive-working high sensitivity electron-beam resist; Japan Pat. No. 80/105,244 discloses the use of poly ethyl cyanoacrylate as a positive-working electron-beam resist; U.S. Pat. No. 4,279,984 discloses the use of homopolymers and copolymers of lower alkyl cyanoacrylates as positive-working high-energy radiation resists; Eur. Pat. No. 48,899 discloses the use of copolymers of alkyl cyanoacrylates as positive-working resists; Japan Pat. No. 82/87,404 discloses the use of polymeric fluoroalkyl cyanoacrylates as electron beam resists; Japan Pat. No. 83/113,933 discloses the use of copolymers of fluoroalkyl acrylates and alkyl cyanoacrylates as resist materials; Japan Pat. No. 83/108,213 dislcoses the use of alkyl cyanoacrylate polymers as positive-working radiation resists; and Japan Pat. No. 83/123,727 discloses the use of alkyl cyanoacrylate polymers and azido crosslinking agents as negative-working radiation resists.
In the prior art, the method of applying the resist materials to the substrate is by spin-coating a solution of the cyanoacrylate polymer onto the substrate. This procedure requires that the polymer is prepared in advance of the application and then dissolved in a suitable solvent. The solution must be filtered to remove particles of dust before application. After application, the coated substrate must be baked to remove residual solvent. Preparation of cyanoacrylate polymers in large quantities is complicated by the high reactivity of the monomers and the strongly exothermic nature of the polymerization.
After polymerization, the polymeric cyanoacrylate must then be precipitated to remove impurities such as initiators and inhibitors, a procedure which consumes large amounts of solvent. The purified polymer must then be redissolved in a suitable solvent for spin-coating. This requirement places a limitation on the types of solvent and polymer molecular weights which may be used. For example, poly methyl cyanoacrylate is soluble in very few solvents, such as nitromethane and dimethylformamide. In general, it is very difficult to spin-coat films of cyanoacrylate polymers with molecular weights in excess of 1,000,000, as stated in U.S. Pat. No. 4,279,984, Col. 2, lines 23-28. However, films formed from polymers below this molecular weight are easily removed by common organic solvents necessitating shortened developing times as stated in U.S. Pat. No. 4,279,984, Col. 3, lines 60-68 and Col. 4, lines 1-9. A further disadvantage of the spin-coating process is that attempts to coat a patterned substrate result in uneven coating thicknesses, i.e., the coating is thicker in depressed regions than in raised regions. One method of overcoming this problem involves the use of multilayer resists. However, applying to or three layers and using two or three image development steps increases the cost and complexity of the lithographic process.
In the prior art, the method of imaging the cyanoacrylate-based resists involves spin-coating a solution of cyanoacrylate polymer onto the substrate and exposing the coated surface to high-energy radiation through a mask or to steered high-energy beams. In the case of positively imaged resists, the exposed portions of the coating are then removed by treatment with solvent, and in the case of a negatively imaged resist the unexposed portions are removed by solvent. This solvent development step inevitably results in some degree of distortion of that portion of the coating which is to remain on the substrate and form a relief image.
Cyanoacrylate monomer vapors have recently been utilized as a fingerprint developer. A detailed discussion of such techniques is included in U.S. Pat. No. 4,550,041.
In a related invention, the applicants here have discovered that resist coatings of cyanoacrylate polymers or other polymers of monomers of the formula CHR.dbd.CXY, where X and Y are strong electron withdrawing groups and R is H or, if X and Y are --CN, alkyl, can be formed by direct deposition of polymer on the substrate from monomer vapors. Such polymeric coatings can be positively imaged in the same manner as prior art cyanoacrylate resist coatings but give much higher molecular weight polymers which diplay significantly improved plasma etch resistance.